1. Field of the Invention
his invention relates generally to the contacting of hydrocarbon vapors with fluidized catalyst particles in a transport riser. More specifically this invention relates to a method of maintaining uniform dispersion of catalyst particles in a fluidized transport riser.
2. Description of the Prior Art
There are a number of continuous cyclical processes employing fluidized solid techniques in which carbonaceous materials are deposited on the solids in the reaction zone and the solids are conveyed during the course of the cycle to another zone where carbon deposits are at least partially removed by combustion in an oxygen-containing medium. The solids from the latter zone are subsequently withdrawn and reintroduced in whole or in part to the reaction zone.
One of the more important processes of this nature is the fluid catalytic cracking (FCC) process for the conversion of relatively high-boiling hydrocarbons to lighter hydrocarbons boiling in the heating oil or gasoline (or lighter) range. The hydrocarbon feed is contacted in one or more reaction zones with the particulate cracking catalyst maintained in a fluidized state under conditions suitable for the conversion of hydrocarbons.
It has been found that the method of contacting the feedstock with the catalyst can dramatically affect the performance of the reaction zone. Modem FCC units use a pipe reactor in the form of a large, usually vertical, riser in which a gaseous medium upwardly transports the catalyst in a fluidized state. Ideally the feed as it enters the riser is instantaneously dispersed throughout a stream of catalyst that is moving up the riser. A complete and instantaneous dispersal of feed across the entire cross section of the riser is not possible, but good results have been obtained by injecting a highly atomized feed into a pre-accelerated stream of catalyst particles. However, the dispersing of the feed throughout the catalyst particles takes some time, so that there is some non-uniform contact between the feed and catalyst as previously described. Non-uniform contacting of the feed and the catalyst exposes portions of the feed to the catalyst for longer periods of time which can in turn produce overcracking and reduce the quality of reaction products.
Much of the effort in improving the hydrocarbon conversion reactions in FCC units has focused on the objective of maximizing the initial dispersal of the hydrocarbon feed into the particulate catalyst suspension. Dividing the feed into small droplets improves dispersion of the feed by increasing the interaction between the liquid and solids. Preferably, the droplet sizes become small enough to permit vaporization of the liquid before it contacts the solids. Techniques such as agitation or shearing are used to atomize a liquid hydrocarbon feed into fine droplets which are then directed at the fluidized solid particles. A variety of methods is known for shearing such liquid streams into fine droplets.
U.S. Pat. No. 3,071,540 discloses a feed injection apparatus for a fluid catalytic cracking unit wherein a high velocity stream of gas, in this case steam, converges around the stream of oil upstream of an orifice through which the mixture of steam and oil is discharged. Initial impact of the steam with the oil stream and subsequent discharge through the orifice atomizes the liquid oil into a dispersion of fine droplets which contact a stream of coaxially flowing catalyst particles.
U.S. Pat. No. 4,434,049 shows a device for injecting a fine dispersion of oil droplets into a fluidized catalyst stream wherein the oil is first discharged through an orifice onto an impact surface located within a mixing tube. The mixing tube delivers a cross flow of steam which simultaneously contacts the liquid. The combined flow of oil and steam exits the conduit through an orifice which atomizes the feed into a dispersion of fine droplets and directs the dispersion into a stream of flowing catalyst particles.
U.S. Pat. No. 5,139,748 issued to Lomas et al. shows the use of radially directed feed injection nozzles to introduce feed into an FCC riser. The nozzles are arranged in a circumferential band about the riser and inject feed toward the center of the riser. The nozzle arrangement and geometry of the riser maintains a substantially open riser cross-section over the feed injection area and downstream riser sections.
Feed atomization, lift-gas and radial injection of feed have been used to more uniformly disperse feed over the cross-section of a riser reaction zone. While it may be possible to obtain a good initial dispersal of the catalyst particles as they contact the vaporized feed, it has been found that as the catalyst passes further up the transport riser it tends to form ribbons or bands of concentrated catalyst that hug the wall of the riser. As feed contacts the hot catalyst, cracking and volumetric expansion of the hydrocarbons causes an increase in the volumetric rate of fluids passing up the riser. A large portion of this volumetric increase occurs immediately downstream of the feed injection point. Downstream of the feed distributor this volumetric expansion occurs in a relatively uncontrolled fashion. The uncontrolled volumetric expansion occurring simultaneously with mixing of catalyst and hydrocarbon feed results in mal-distribution that adversely effects the quantity and quality of the products obtained from the cracking reaction. This maldistribution is believed to be caused by turbulent back mixing as well as quiescent zones in the riser section immediately downstream of the feed injection point.
These flowing ribbons of catalyst cause non-uniform regions of density and result in uneven contacting of the catalyst with the hydrocarbon feed. These ribbons of catalyst produce slippage between the fluid stream and the catalyst stream that further increase the nonuniformity of the contacting between catalyst and vapors. All of these phenomena contribute to an increase in the non-uniformity of the contacting between the catalyst and the gas.
It is known to use screens and obstacles in conduits that transport particulate material. U.S. Pat. No. 4,071,573 shows the use of screens to disperse bubbles that form in the dense phase transport and contacting of catalyst and a feedstream. U.S. Pat. No. 3,799,868 discloses a riser for the dilute phase contacting of gaseous hydrocarbons that blocks the center of the riser to eliminate the central area of the riser as a potentially more dilute flow area. It is also taught to intensify gas solids contacting by placing large obstacles in the path of circulating gas solids stream. "Hydrodynamics of a Pilot-Plant Scale Regularly Packed Circulating Fluidized Bed," was presented at an AIchE Symposium Series by A. G. J. van der Ham, W. Prins, and W. P. M. van Swaaij of Chemical Reaction Eng. Labs, Chemical Eng. Dept., Twente University, The Netherlands in 1993.